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Moon and Spica

May 17, 2016

The Moon’s gravity pulls on the water in Earth’s oceans. That creates “bulges” in the oceans that move as Earth rotates on its axis, creating the daily cycle of high and low tides.

Tides aren’t limited to planets and moons, though — they’re also found on stars. In fact, a good example is a star system that keeps company with the gibbous Moon late tonight: Spica, the leading light of Virgo. It’s to the lower left of the Moon at nightfall, and closer to its left as they set in the wee hours of the morning.

Spica actually consists of two stars, each of which is much bigger, brighter, and heavier than the Sun. They’re so close together that they orbit each other once every four days.

At that range, each star exerts a powerful gravitational pull on the other. That pull is strongest on the sides of the stars that face each other, just as the Moon’s pull is strongest on the side of Earth that faces it. The difference in the pull on the two sides of each star causes the stars to bulge toward each other. As a result, the stars are slightly egg-shaped, with the tapered ends pointing at each other.

We can’t see this effect directly because the stars blur together to form a single point of light. But we can see it indirectly. As the two stars orbit each other, we see them from different angles. When we look at the system from the side, where the bulges are most obvious, Spica grows a tiny bit brighter — the result of giant tides on giant stars.

Script by Damond Benningfield

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